Magnetic resonance imaging interference immune device

ABSTRACT

A voltage compensation unit reduces the effects of induced voltages upon a device having a single wire line. The single wire line has balanced characteristic impedance. The voltage compensation unit includes a tunable compensation circuit connected to the wire line. The tunable compensation circuit applies supplemental impedance to the wire line. The supplemental impedance causes the characteristic impedance of the wire line to become unbalanced, thereby reducing the effects of induced voltages caused by changing magnetic fields.

PRIORITY INFORMATION

This application claims priority under 35 U.S.C. §119(e) from U.S.Provisional Patent Application, Ser. No. 60/482,177, filed on Jun. 24,2003. The entire content of U.S. Provisional Patent Application, Ser.No. 60/482,177 is hereby incorporated by reference.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The subject matter of co-pending U.S. patent application Ser. No.09/885,867, filed on Jun. 20, 2001, entitled “Controllable, WearableMRI-Compatible Cardiac Pacemaker With Pulse Carrying Photonic CatheterAnd VOO Functionality”; co-pending U.S. patent application Ser. No.09/885,868, filed on Jun. 20, 2001, entitled “Controllable, WearableMRI-Compatible Cardiac Pacemaker With Power Carrying Photonic CatheterAnd VOO Functionality”; co-pending U.S. patent application Ser. No.10/037,513, filed on Jan. 4, 2002, entitled “Optical Pulse Generator ForBattery Powered Photonic Pacemakers And Other Light Driven MedicalStimulation Equipment”; co-pending U.S. patent application Ser. No.10/037,720, filed on Jan. 4, 2002, entitled “Opto-Electric CouplingDevice For Photonic Pacemakers. And Other Opto-Electric MedicalStimulation Equipment”; co-pending U.S. patent application Ser. No.09/943,216, filed on Aug. 30, 2001, entitled “Pulse width Cardiac PacingApparatus”; co-pending U.S. patent application Ser. No. 09/964,095,filed on Sep. 26, 2001, entitled “Process for Converting Light”; andco-pending U.S. patent application Ser. No. 09/921,066, filed on Aug. 2,2001, entitled “MRI-Resistant Implantable Device”. The entire contentsof each of the above noted co-pending U.S. patent applications (Ser.Nos. 09/885,867; 09/885,868; 10/037,513; 10/037,720; 09/943,216;09/964,095; and 09/921,066) are hereby incorporated by reference.

FIELD OF THE PRESENT INVENTION

The present invention is directed to a device for protecting a patient,physician, and/or electronic components in an electrical deviceimplanted or partially implanted within the patient. More particularly,the present invention is directed to a device for protecting theconductive parts of the electrical device from current and voltagesurges induced by magnetic resonance imaging systems'oscillatingmagnetic fields.

BACKGROUND OF THE PRESENT INVENTION

Magnetic resonance imaging (“MRI”) has been developed as an imagingtechnique adapted to obtain both images of anatomical features of humanpatients as well as some aspects of the functional activities andcharacteristics of biological tissue. These images have medicaldiagnostic value in determining the state of the health of the tissueexamined. Unlike the situation with fluoroscopic imaging, a patientundergoing magnetic resonance imaging procedure may remain in the activeimaging system for a significant amount of time, e.g. a half-hour ormore, without suffering any adverse effects.

In an MRI process, a patient is typically aligned to place the portionof the patient's anatomy to be examined in the imaging volume of the MRIapparatus. Such an MRI apparatus typically comprises a primaryelectromagnet for supplying a constant magnetic field (B₀) which, byconvention, is along the z-axis and is substantially homogeneous overthe imaging volume and secondary electromagnets that can provide linearmagnetic field gradients along each of three principal Cartesian axes inspace (generally x, y, and z, or x₁, x₂ and x₃, respectively). The MRIapparatus also comprises one or more radio frequency coils that provideexcitation and detection of the MRI induced signals in the patient'sbody.

The gradient fields are switched ON and OFF at different rates dependingon the MRI scan sequence used. In some cases, this may result in achanging magnetic field on the order of dB/dt=50 T/s. The frequency thata gradient field may be turned ON can be between 200 Hz to about 300kHz.

For a single loop with a fixed area, Lenz's law can be stated as:EMF=−A∘dB/dtwhere A is the area vector, B is the magnetic field vector, and “∘” isthe vector scalar product. This equation indicates that anelectro-motive-force (EMF) is developed in any loop that encircles achanging magnetic field.

In an MRI system, there is applied to the biological sample (patient) aswitched gradient field in all 3 coordinate directions (x-, y-,z-directions). If the patient has an implanted heart pacemaker (or otherimplanted devices having conductive components) the switched gradientmagnetic fields (an alternating magnetic field) may cause:

-   -   1. Erroneous signals to be induced/generated in a sensing lead        or device or circuit;    -   2. Damage to electronics; and/or    -   3. Harmful stimulation of tissue, e.g. heart muscle, nerves,        etc.

As noted above, the use of the MRI process with patients who haveimplanted medical assist devices; such as cardiac assist devices orimplanted insulin pumps; often presents problems. As is known to thoseskilled in the art, implantable devices (such as implantable pulsegenerators (IPGs) and cardioverter/defibrillator/pacemakers (CDPs)) aresensitive to a variety of forms of electromagnetic interference (EMI)because these enumerated devices include sensing and logic systems thatrespond to low-level electrical signals emanating from the monitoredtissue region of the patient. Since the sensing systems and conductiveelements of these implantable devices are responsive to changes in localelectromagnetic fields, the implanted devices are vulnerable to externalsources of severe electromagnetic noise, and in particular, toelectromagnetic fields emitted during the magnetic resonance imaging(MRI) procedure. Thus, patients with implantable devices are generallyadvised not to undergo magnetic resonance imaging (MRI) procedures.

To more appreciate the problem, the use of implantable cardiac assistdevices during a MRI process will be briefly discussed.

The human heart may suffer from two classes of rhythmic disorders orarrhythmias: bradycardia and tachyarrhythmia. Bradycardia occurs whenthe heart beats too slowly, and may be treated by a common implantablepacemaker delivering low voltage (about 3 V) pacing pulses.

The common implantable pacemaker is usually contained within ahermetically sealed enclosure, in order to protect the operationalcomponents of the device from the harsh environment of the body, as wellas to protect the body from the device.

The common implantable pacemaker operates in conjunction with one ormore electrically conductive leads, adapted to conduct electricalstimulating pulses to sites within the patient's heart, and tocommunicate sensed signals from those sites back to the implanteddevice.

Furthermore, the common implantable pacemaker typically has a metal caseand a connector block mounted to the metal case that includesreceptacles for leads which may be used for electrical stimulation orwhich may be used for sensing of physiological signals. The battery andthe circuitry associated with the common implantable pacemaker arehermetically sealed within the case. Electrical interfaces are employedto connect the leads outside the metal case with the medical devicecircuitry and the battery inside the metal case.

Electrical interfaces serve the purpose of providing an electricalcircuit path extending from the interior of a hermetically sealed metalcase to an external point outside the case while maintaining thehermetic seal of the case. A conductive path is provided through theinterface by a conductive pin that is electrically insulated from thecase itself.

Such interfaces typically include a ferrule that permits attachment ofthe interface to the case, the conductive pin, and a hermetic glass orceramic seal that supports the pin within the ferrule and isolates thepin from the metal case.

A common implantable pacemaker can, under some circumstances, besusceptible to electrical interference such that the desiredfunctionality of the pacemaker is impaired. For example, commonimplantable pacemaker requires protection against electricalinterference from electromagnetic interference (EMI), defibrillationpulses, electrostatic discharge, or other generally large voltages orcurrents generated by other devices external to the medical device. Asnoted above, more recently, it has become crucial that cardiac assistsystems be protected from magnetic-resonance imaging sources.

Such electrical interference can damage the circuitry of the cardiacassist systems or cause interference in the proper operation orfunctionality of the cardiac assist systems. For example, damage mayoccur due to high voltages or excessive currents introduced into thecardiac assist system.

Moreover, problems are realized when the placement of the implant isnext to particular organs. For example, when a pacemaker is placed inthe upper chest and the lead tip is placed into the heart, a loop (anelectrical loop) is created. A changing magnetic field (the switchedgradient field) over the area of the loop (through the area of the loop)will cause an induced voltage (and current) across the heart. Thisinduced voltage (current) can stimulate the heart inappropriately andcan cause heart damage or death.

Therefore, it is desirable to provide a medical device or system thatreduces or eliminates the undesirable effects of changing magneticfields from an MRI system on the medical devices and/or patientsundergoing medical procedures or that have temporary or permanentimplanted materials and/or devices with conducting components.

SUMMARY OF THE PRESENT INVENTION

A first aspect of the present invention is a voltage compensation unitfor reducing the effects of induced voltages upon a device to a safelevel. The voltage compensation unit includes a sensing circuit to sensevoltages induced in conductive components of the device, the voltagesbeing induced by changing magnetic fields and a compensation circuit,operatively connected to the sensing circuit and responsive thereto, toprovide opposing voltages to the device to reduce the effects of inducedvoltages caused by changing magnetic fields.

A second aspect of the present invention is a voltage compensation unitfor reducing the effects of induced voltages upon a tissue invasivemedical tool to a safe level. The voltage compensation unit includes asensing circuit to sense voltages induced in conductive components ofthe medical tool, the voltages being induced by changing magneticfields; a compensation circuit, operatively connected to the sensingcircuit and responsive thereto, to provide opposing voltages to themedical tool to reduce the effects of induced voltages caused bychanging magnetic fields; and a connection device to provide anelectrical connection between the sensing circuit and the compensationcircuit and the medical tool.

A third aspect of the present invention is a voltage compensation unitfor reducing the effects of induced voltages upon a device to a safelevel. The voltage compensation unit includes a communication circuit,communicatively linked to a MRI system, to receive informationassociated with a start and end of an application of changing magneticfields produced by the MRI system and a compensation circuit,operatively connected to the communication circuit and responsivethereto, to synchronize application of opposing voltages to the devicewith the sensed changing magnetic fields, the opposing voltages reducingthe effects of induced voltages caused by the changing magnetic fields.

A fourth aspect of the present invention is a voltage compensation unitfor reducing the effects of induced voltages upon a device to a safelevel. The voltage compensation unit includes a communication circuit,communicatively linked to a MRI system, to receive informationassociated with a start and end of an application of changing magneticfields produced by the MRI system and a compensation circuit,operatively connected to the communication circuit and responsivethereto, to apply opposing voltages to the device, the opposing voltagesreducing the effects of induced voltages caused by the changing magneticfields.

A fifth aspect of the present invention is a voltage compensation unitfor reducing the effects of induced voltages upon a device having asingle wire line, the single wire line having a balanced characteristicimpedance. The voltage compensation unit includes a tunable compensationcircuit, operatively connected to the wire line, to apply supplementalimpedance to the wire line, the supplemental impedance causing thecharacteristic impedance of the wire line to become unbalanced, therebyreducing the effects of induced voltages caused by changing magneticfields.

Another aspect of the present invention is a system for reducing theeffects of MRI induced signals to a safe level. The system includes amedical device wherein the medical device has a housing havingelectronic circuitry therein, a first lead to provide an electrical pathfor a stimulation signal generated by the electronic circuitry to beapplied to a desired tissue region, a second lead to provide anelectrical path for a sensed physiological condition of the desiredtissue region to be communicated to the electronic circuitry, and athird lead to provide an electrical ground. The system also includes adiode, operatively connected to the first lead, to significantly reduceMRI induced signals from traveling along the first lead to theelectronic circuitry.

A further aspect of the present invention is a system for reducing theeffects of MRI induced signals to a safe level. The system includes aMRI system; a medical device; and a transceiver to provide communicationbetween the MRI system and the medical device. The medical device has ahousing having electronic circuitry therein, a bi-directional lead toprovide an electrical path for a stimulation signal generated by theelectronic circuitry to be applied to a desired tissue region and toprovide an electrical path for a sensed physiological condition of thedesired tissue region to be communicated to the electronic circuitry,and a lead to provide an electrical ground. The medical device indicatesto the MRI system, through the transceiver, when the stimulation signalwill be applied to the desired tissue region. The MRI system, inresponse to the indication from the medical device of when thestimulation signal will be applied to the desired tissue region,terminates a production of MRI switched gradient fields.

A further aspect of the present invention is a system for reducing theeffects of MRI induced signals to a safe level. The system includes amedical device wherein the medical device has a housing havingelectronic circuitry therein, and leads to provide an electrical pathfor a stimulation signal generated by the electronic circuitry to beapplied to a desired tissue region and to provide an electrical path fora sensed physiological condition of the desired tissue region to becommunicated to the electronic circuitry, a sensor to sense applicationof switched MRI gradient fields, and an electronic component,operatively connected to the leads, to significantly reduce MRI inducedsignals from traveling along the leads to the electronic circuitry, anda switch, operatively connected to the sensor and the electroniccomponent, to operatively connect the electronic component to the leadswhen the sensor senses the application of switched MRI gradient fieldsand to operatively disconnect the electronic component from the leadswhen the sensor fails to sense the application of switched MRI gradientfields.

A further aspect of the present invention is an electrical leadcomponent for a medical device that reduces the effects of MRI inducedsignals to a safe level. The electrical lead component includes amedical device electrical lead capable of providing an electrical pathto a desired tissue region and a coil that generates a MRI switchedgradient field induced current opposite to that which would be inducedby the MRI switched gradient fields in the medical device electricallead so as to reduce voltages induced by the MRI switched gradientfields to a safe level.

A further aspect of the present invention is an electrical leadcomponent for a medical device that reduces the effects of MRI inducedsignals to a safe level. The electrical lead component includes amedical device electrical lead capable of providing an electrical pathto a desired tissue region and a plurality of coils, each coilgenerating a MRI switched gradient field induced current such acombination of the MRI switched gradient field induced currents providea combined current that is opposite to that which would be induced bythe MRI switched gradient fields in the medical device electrical leadso as to reduce voltages induced by the MRI switched gradient fields toa safe level.

A further aspect of the present invention is an electrical leadcomponent for a medical device that reduces the effects of MRI inducedsignals to a safe level. The electrical lead component includes amedical device electrical lead capable of providing an electrical pathto a desired tissue region and three orthogonally planar coils, eachcoil generating a MRI switched gradient field induced current such acombination of the MRI switched gradient field induced currents providea combined current that is opposite to that which would be induced bythe MRI switched gradient fields in the medical device electrical leadso as to reduce voltages induced by the MRI switched gradient fields toa safe level.

A further aspect of the present invention is an electrical leadcomponent for a medical device that reduces the effects of MRI inducedsignals to a safe level. The electrical lead component includes amedical device electrical lead capable of providing an electrical pathto a desired tissue region; a plurality of coils, each coil generating aMRI switched gradient field induced current; a sensor to measure astrength of voltages induced by the MRI switched gradient fields; and aswitching device, operatively connected to the sensor and plurality ofcoils, to operatively connect a number of the plurality of coils inresponse to the measured strength of voltages induced by the MRIswitched gradient fields such that a combination of the MRI switchedgradient field induced currents produced by the number of operativelyconnected switches provide a combined current that is opposite to thatwhich would be induced by the MRI switched gradient fields in themedical device electrical lead so as to reduce voltages induced by theMRI switched gradient fields to a safe level.

A further aspect of the present invention is an electrical leadcomponent for a medical device that reduces the effects of MRI inducedsignals to a safe level. The electrical lead component includes amedical device electrical lead capable of providing an electrical pathto a desired tissue region; three orthogonally planar coils, each coilgenerating a MRI switched gradient field induced current; a sensor tomeasure a strength of voltages induced by the MRI switched gradientfields; and a switching device, operatively connected to the sensor andthe coils, to operatively connect a number of the coils in response tothe measured strength of voltages induced by the MRI switched gradientfields such that a combination of the MRI switched gradient fieldinduced currents produced by the number of operatively connectedswitches provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

A further aspect of the present invention is an electrical leadcomponent for a medical device that reduces the effects of MRI inducedsignals to a safe level. The electrical lead component includes amedical device electrical lead capable of providing an electrical pathto a desired tissue region; a plurality of coils, each coil generating aMRI switched gradient field induced current; a transceiver to receive asignal indicating a number of coils to be connected; and a switchingdevice, operatively connected to the transceiver and plurality of coils,to operatively connect a number of the plurality of coils in response tothe received signal indicating the number of coils to be connected suchthat a combination of the MRI switched gradient field induced currentsproduced by the number of operatively connected switches provide acombined current that is opposite to that which would be induced by theMRI switched gradient fields in the medical device electrical lead so asto reduce voltages induced by the MRI switched gradient fields to a safelevel.

A further aspect of the present invention is an electrical leadcomponent for a medical device that reduces the effects of MRI inducedsignals to a safe level. The electrical lead component includes amedical device electrical lead capable of providing an electrical pathto a desired tissue region; three orthogonally planar coils, each coilgenerating a MRI switched gradient field induced current; a transceiverto receive a signal indicating a number of coils to be connected; and aswitching device, operatively connected to the transceiver and thecoils, to operatively connect a number of the coils in response to thereceived signal indicating the number of coils to be connected such thata combination of the MRI switched gradient field induced currentsproduced by the number of operatively connected switches provide acombined current that is opposite to that which would be induced by theMRI switched gradient fields in the medical device electrical lead so asto reduce voltages induced by the MRI switched gradient fields to a safelevel.

A further aspect of the present invention is a medical device thatreduces the effects of MRI induced signals to a safe level. The medicaldevice includes a medical device capable of providing medical treatmentto a desired tissue region and a coil that generates a MRI switchedgradient field induced current opposite to that which would be inducedby the MRI switched gradient fields in the medical device so as toreduce voltages induced by the MRI switched gradient fields to a safelevel.

A further aspect of the present invention is a medical device thatreduces the effects of MRI induced signals to a safe level. The medicaldevice includes a medical device capable of providing medical treatmentto a desired tissue region and a plurality of coils, each coilgenerating a MRI switched gradient field induced current such acombination of the MRI switched gradient field induced currents providea combined current that is opposite to that which would be induced bythe MRI switched gradient fields in the medical device so as to reducevoltages induced by the MRI switched gradient fields to a safe level.

A further aspect of the present invention is a medical device thatreduces the effects of MRI induced signals to a safe level. The medicaldevice includes a medical device capable of providing medical treatmentto a desired tissue region and three orthogonally planar coils, eachcoil generating a MRI switched gradient field induced current such acombination of the MRI switched gradient field induced currents providea combined current that is opposite to that which would be induced bythe MRI switched gradient fields in the medical device so as to reducevoltages induced by the MRI switched gradient fields to a safe level.

A further aspect of the present invention is a medical device thatreduces the effects of MRI induced signals to a safe level. The medicaldevice includes a medical device capable of providing medical treatmentto a desired tissue region; a plurality of coils, each coil generating aMRI switched gradient field induced current; a sensor to measure astrength of voltages induced by the MRI switched gradient fields; and aswitching device, operatively connected to the sensor and plurality ofcoils, to operatively connect a number of the plurality of coils inresponse to the measured strength of voltages induced by the MRIswitched gradient fields such that a combination of the MRI switchedgradient field induced currents produced by the number of operativelyconnected switches provide a combined current that is opposite to thatwhich would be induced by the MRI switched gradient fields in themedical device so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

A further aspect of the present invention is a medical device thatreduces the effects of MRI induced signals to a safe level. The medicaldevice includes a medical device capable of providing medical treatmentto a desired tissue region; three orthogonally planar coil, each coilgenerating a MRI switched gradient field induced current; a sensor tomeasure a strength of voltages induced by the MRI switched gradientfields; and a switching device, operatively connected to the sensor andplurality of coils, to operatively connect a number of the plurality ofcoils in response to the measured strength of voltages induced by theMRI switched gradient fields such that a combination of the MRI switchedgradient field induced currents produced by the number of operativelyconnected switches provide a combined current that is opposite to thatwhich would be induced by the MRI switched gradient fields in themedical device so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

A further aspect of the present invention is a medical device thatreduces the effects of MRI induced signals to a safe level. The medicaldevice includes a medical device capable of providing medical treatmentto a desired tissue region; a plurality of coils, each coil generating aMRI switched gradient field induced current; a transceiver to receive asignal indicating a number of coils to be connected; and a switchingdevice, operatively connected to the transceiver and the coils, tooperatively connect a number of the coils in response to the receivedsignal indicating the number of coils to be connected such that acombination of the MRI switched gradient field induced currents producedby the number of operatively connected switches provide a combinedcurrent that is opposite to that which would be induced by the MRIswitched gradient fields in the medical device electrical lead so as toreduce voltages induced by the MRI switched gradient fields to a safelevel.

A further aspect of the present invention is a medical device thatreduces the effects of MRI induced signals to a safe level. The medicaldevice includes a medical device capable of providing medical treatmentto a desired tissue region; three orthogonally planar coil, each coilgenerating a MRI switched gradient field induced current; a transceiverto receive a signal indicating a number of coils to be connected; and aswitching device, operatively connected to the transceiver and thecoils, to operatively connect a number of the coils in response to thereceived signal indicating the number of coils to be connected such thata combination of the MRI switched gradient field induced currentsproduced by the number of operatively connected switches provide acombined current that is opposite to that which would be induced by theMRI switched gradient fields in the medical device electrical lead so asto reduce voltages induced by the MRI switched gradient fields to a safelevel.

A further aspect of the present invention is a voltage control unit thatreduces the effects of MRI induced signals upon a medical device to asafe level. The voltage control unit includes a coil that generates aMRI switched gradient field induced current opposite to that which wouldbe induced by the MRI switched gradient fields in the medical device soas to reduce voltages induced by the MRI switched gradient fields to asafe level.

A further aspect of the present invention is a voltage control unit thatreduces the effects of MRI induced signals upon a medical device to asafe level. The voltage control unit includes a plurality of coils, eachcoil generating a MRI switched gradient field induced current such acombination of the MRI switched gradient field induced currents providea combined current that is opposite to that which would be induced bythe MRI switched gradient fields in the medical device so as to reducevoltages induced by the MRI switched gradient fields to a safe level.

A further aspect of the present invention is a voltage control unit thatreduces the effects of MRI induced signals upon a medical device to asafe level. The voltage control unit includes three orthogonally planarcoils, each coil generating a MRI switched gradient field inducedcurrent such a combination of the MRI switched gradient field inducedcurrents provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical device soas to reduce voltages induced by the MRI switched gradient fields to asafe level.

A further aspect of the present invention is a voltage control unit thatreduces the effects of MRI induced signals upon a medical device to asafe level. The voltage control unit includes a plurality of coils, eachcoil generating a MRI switched gradient field induced current; a sensorto measure a strength of voltages induced by the MRI switched gradientfields; and a switching device, operatively connected to the sensor andplurality of coils, to operatively connect a number of the plurality ofcoils in response to the measured strength of voltages induced by theMRI switched gradient fields such that a combination of the MRI switchedgradient field induced currents produced by the number of operativelyconnected switches provide a combined current that is opposite to thatwhich would be induced by the MRI switched gradient fields in themedical device so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

A further aspect of the present invention is a voltage control unit thatreduces the effects of MRI induced signals upon a medical device to asafe level. The voltage control unit includes three orthogonally planarcoil, each coil generating a MRI switched gradient field inducedcurrent; a sensor to measure a strength of voltages induced by the MRIswitched gradient fields; and a switching device, operatively connectedto the sensor and plurality of coils, to operatively connect a number ofthe plurality of coils in response to the measured strength of voltagesinduced by the MRI switched gradient fields such that a combination ofthe MRI switched gradient field induced currents produced by the numberof operatively connected switches provide a combined current that isopposite to that which would be induced by the MRI switched gradientfields in the medical device so as to reduce voltages induced by the MRIswitched gradient fields to a safe level.

A further aspect of the present invention is a voltage control unit thatreduces the effects of MRI induced signals upon a medical device to asafe level. The voltage control unit includes a plurality of coils, eachcoil generating a MRI switched gradient field induced current; atransceiver to receive a signal indicating a number of coils to beconnected; and a switching device, operatively connected to thetransceiver and the coils, to operatively connect a number of the coilsin response to the received signal indicating the number of coils to beconnected such that a combination of the MRI switched gradient fieldinduced currents produced by the number of operatively connectedswitches provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

A further aspect of the present invention is a voltage control unit thatreduces the effects of MRI induced signals upon a medical device to asafe level. The voltage control unit includes three orthogonally planarcoil, each coil generating a MRI switched gradient field inducedcurrent; a transceiver to receive a signal indicating a number of coilsto be connected; and a switching device, operatively connected to thetransceiver and the coils, to operatively connect a number of the coilsin response to the received signal indicating the number of coils to beconnected such that a combination of the MRI switched gradient fieldinduced currents produced by the number of operatively connectedswitches provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

A further aspect of the present invention is a lead for medicalapplications that reduces the effects of MRI induced signals to a safelevel. The lead includes two coiled conductive strands forming aspring-like configuration such that current flows over a surfacethereof, through contact points between adjacent loops of the coiledconductive strands, and an insulating coating formed over a portion ofthe two coiled conductive strands such that an inline inductive elementis formed, the current flowing along a curvature of the two coiledconductive strands in the insulating coated portion of two coiledconductive strands.

A further aspect of the present invention is a lead for medicalapplications that reduces the effects of MRI induced signals to a safelevel. The lead includes two coiled conductive strands forming aspring-like configuration such that current flows over a surfacethereof, through contact points between adjacent loops of the coiledconductive strands, and an adjustable resistive material formed over aportion of the two coiled conductive strands such that an inlineinductive element is formed, the current flowing along a curvature ofthe two coiled conductive strands in the adjustable resistive materialportion of two coiled conductive strands, an inductance of the inlineinductive element being adjusted by adjusting the resistive propertiesof the adjustable resistive material.

A further aspect of the present invention is a voltage compensation unitfor reducing the effects of induced voltages upon a medical device to asafe level. The voltage compensation unit includes a connection deviceto provide an electrical connection to the medical device; a sensingcircuit to voltages of conductive components in the medical device; anda compensation circuit, operatively connected to the sensing circuit andresponsive thereto, to provide opposing voltages to the medical deviceto reduce the effects of induced voltages caused by changing magneticfields.

A further aspect of the present invention is a voltage compensation unitfor reducing the effects of induced voltages upon a medical device to asafe level. The voltage compensation unit includes a connection deviceto provide an electrical connection to the medical device; a sensingcircuit to detect changing magnetic fields; and a compensation circuit,operatively connected to the sensing circuit and responsive thereto, tosynchronize application of opposing voltages to the medical device withthe sensed changing magnetic fields, the opposing voltages reducing theeffects of induced voltages caused by the changing magnetic fields.

A further aspect of the present invention is a voltage compensation unitfor reducing the effects of induced voltages upon a medical device to asafe level. The voltage compensation unit includes a connection deviceto provide an electrical connection to the medical device; acommunication circuit, communicatively linked to a MRI system, toreceive information associated with a start and end of an application ofchanging magnetic fields produced by the MRI system; and a compensationcircuit, operatively connected to the communication circuit andresponsive thereto, to synchronize application of opposing voltages tothe medical device with the sensed changing magnetic fields, theopposing voltages reducing the effects of induced voltages caused by thechanging magnetic fields.

A further aspect of the present invention is a voltage compensation unitfor reducing the effects of induced voltages upon a medical device to asafe level. The voltage compensation unit includes a connection deviceto provide an electrical connection to the medical device; acommunication circuit, communicatively linked to a MRI system, toreceive information associated with a start and end of an application ofchanging magnetic fields produced by the MRI system; and a compensationcircuit, operatively connected to the communication circuit andresponsive thereto, to apply opposing voltages to the medical device,the opposing voltages reducing the effects of induced voltages caused bythe changing magnetic fields.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may take form in various components andarrangements of components, and in various steps and arrangements ofsteps. The drawings are only for purposes of illustrating a preferredembodiment and are not to be construed as limiting the presentinvention, wherein:

FIG. 1 is a schematic of an implanted pacemaker arrangement in a body;

FIG. 2 is a schematic of a pacemaker lead comprising three conductivestrands;

FIG. 3 is a schematic of a sensing system used with a pacemaker;

FIG. 4 illustrates an embodiment of a pacemaker canister according tothe concepts of the present invention;

FIG. 5 illustrates another embodiment of a pacemaker canister accordingto the concepts of the present invention;

FIG. 6 illustrates a further embodiment of a pacemaker canisteraccording to the concepts of the present invention;

FIG. 7 is an illustration of inductive currents in conductor loops;

FIG. 8 is an illustration of canceling inductive currents in conductorloops according to the concepts of the present invention;

FIG. 9 is a schematic of an embodiment of a pacemaker lead utilizinginductive loops according to the concepts of the present invention;

FIG. 10 is a schematic of an embodiment of inductive loops in apacemaker canister according to the concepts of the present invention;

FIG. 11 is a schematic of an embodiment of inductive loops around apacemaker canister according to the concepts of the present invention;

FIG. 12 illustrates of an embodiment of a medical device with anexternal voltage cancellation unit according to the concepts of thepresent invention;

FIG. 13 illustrates of another embodiment of a medical device with anexternal voltage cancellation unit according to the concepts of thepresent invention;

FIG. 14 illustrates a portion of coiled leads used in a medial deviceaccording to the concepts of the present invention;

FIG. 15 illustrates another embodiment of a portion of coiled leads usedin a medial device according to the concepts of the present invention;

FIG. 16 illustrates a further embodiment of a portion of coiled leadsused in a medial device according to the concepts of the presentinvention;

FIG. 17 illustrates another embodiment of a portion of coiled leads usedin a medial device according to the concepts of the present invention;

FIG. 18 illustrates a circuit diagram representing a guide wire with anunbalancing impedance circuit according to the concepts of the presentinvention;

FIG. 19 illustrates another embodiment of a circuit diagram representinga guide wire with an unbalancing impedance circuit according to theconcepts of the present invention;

FIG. 20 illustrates a balun used in conjunction with a guide wireaccording to the concepts of the present invention; and

FIG. 21 is a circuit diagram representing a capacitance unbalanced balununit according to the concepts of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention will be described in connection with preferredembodiments; however, it will be understood that there is no intent tolimit the present invention to the embodiments described herein. On thecontrary, the intent is to cover all alternatives, modifications, andequivalents as may be included within the spirit and scope of thepresent invention as defined by the appended claims.

For a general understanding of the present invention, reference is madeto the drawings. In the drawings, like reference have been usedthroughout to designate identical or equivalent elements. It is alsonoted that the various drawings illustrating the present invention arenot drawn to scale and that certain regions have been purposely drawndisproportionately so that the features and concepts of the presentinvention could be properly illustrated.

FIG. 1 is a schematic showing a typical pacemaker arrangement 100. Thepacemaker comprises a pulse generator canister 102 housing a powersupply (not shown) and electronic components (not shown) for sensing andproducing electrical pacing pulses. The pulse generator canister 102 hasconnected to it insulated conductive leads 104 that pass through thebody (not shown) and into the heart 106. Conventional bipolar pacemakerleads have two conductive strands, one for pacing and sensing, and theother for ground. The path of the leads 104 is generally not straight.The leads 104 have one or more electrodes 112 in contact with the heart106. The direct line 108 from the heart 106, where the electrodes 112are placed, to the generator canister 102 represents a conductive pathcomprising body tissue (not shown) and fluids (not shown). The completedloop from the pacemaker canister 102, through the leads 104, and back tothe pacemaker canister 102 along the path 108 is subject to Lenz's law.That is, a changing magnetic field 110 through the area enclosed by thecompleted loop (from the pacemaker canister 102, through the leads 104,and back to the pacemaker canister 102 along the path 108) can induceunwanted voltages in the leads 104 and across the heart 106.

In one embodiment of the present invention, and referring to FIG. 1, thepacemaker canister 102 is made out of a non-conductive material. Inanother embodiment, the canister 102 is coated or covered with variousnon-conductive insulating materials. This increases the overallresistance of the conductive path loop and thus reduces the voltageacross the tissue between electrodes 112 and the canister 102.

Using a three-strand lead design allows for the separation of the pacingsignals from the sensing signals and allows for different filteringtechniques to be utilized on each separate conductive strand: one strandfor the pacing signal for stimulating the heart, one conductive strandfor the sensing of the heart's electrical state, pre-pulse, ecg, etc.,and one strand for the ground path. Current bi-polar designs use onlytwo conductive strands. This means that the pacing and the sensingsignals are carried on the same strand.

For example, in conventional bipolar pacemaker leads, the pacing signalgoes “down” (from generator canister to heart) the pacing lead(conductive strand) while the sensing signal travels “up” (from heart togenerator canister) the pacing lead. This is the “standard” bipolarpacing setup. If a filter is added to the pacing/sensing strand to blockthe switch gradient induced signal caused by a MRI system, the pacingpulse/signal must travel through the filter, thereby distorting thepacing pulse.

According to the concepts of the present invention, by adding a thirdconductive strand, a diode, for example, can be put on the pacing strandand one or more filters can be put on the sensing strand. The filters onthe sensing lead may be at the distal end of the pacemaker lead or inthe generator canister. Thus, by using separate strands, the presentinvention is able to utilize different kinds of filters (radio-frequencyfilters, high/low pass filters, notch filters, etc.) or otherelectronics in conjunction with each strand depending on the differentsignal characteristics and/or signal direction along the conductivestrand.

FIG. 2 shows a schematic of a pacemaker arrangement 120 including agenerator canister 122 containing a pacing pulse generator (not shown),sensing electronics (not shown) and other electronic components (notshown). Attached to the generator canister 122 is a lead assembly 140having three conductive strands 124, 126, and 128 through lumen 138.Each of the conductive strands 124, 126, and 128 pass through the distaltip 142 of the lead assembly 140 to exposed electrodes 132, 134, and136, respectively. The exposed electrodes 132, 134, and 136 are placedin contact with or next to the heart.

Conductive strand 124 and electrode 132 are used to deliver pulses tothe heart from a pacing generator within the canister 122. Conductivestrand 126 and electrode 134 are used as a ground. Conductive strand 128and electrode 136 are utilized for sensing the electrical signalsgenerated by the heart. In this way, the sensing functionality ofpacemakers can be separated from the delivery of pacing pulses.

To block any induced voltage signals from the MRI system's changingmagnetic fields (the radio-frequency or the gradient fields) frompropagating along the conductive pulse delivery strand 124, a diode 130is inserted into the conductive strand 124 near the distal tip of thelead assembly 142. It is noted that the diode 130 can also be is placedin the generator canister 122.

With respect to FIG. 2, other electronic components (i.e.radio-frequency chocks, notch filters, etc.) may be placed into theother conductive strands 126 and 128 shown as by components 146 and 144,respectively. It is noted that these optional electronic components 146and 144 can be placed in the generator canister 122.

Optional electronic components 146 and 144 are used to block orsignificantly reduce any unwanted induced signals caused by the MRIsystem from passing along conductive strands 126 and 128 respectivelywhile allowing the desired sensing signals from the heart to pass alongconductive strand 126 to electronics in the generator canister 122.

FIG. 3 is a schematic of an embodiment of the present invention. Asillustrated in FIG. 3, a patient 162 is located within an MRI system168, wherein the patient 162 has an implanted heart pacemaker pulsegenerator canister 164. A surface sensor/transceiver 166 is placed onthe exterior of the patient's body 162 over or near the location of theimplanted pacemaker generator 164. The sensor/transceiver 166 is incommunication with the MRI system 168 via communication line 170, whichmay be an MRI safe cable such as a fiber optical cable. Additionally,the sensor/transceiver 166 is in communication with the implantedpacemaker pulse generator canister 164. The means of communicationbetween the sensor/transceiver 166 and the implanted pacemaker generator164 may be acoustic, optical, or other means that do not interfere withthe imaging capabilities or image quality of the MRI system. The signalsmay be digital or analog.

Moreover, with respect to this embodiment of the present invention, atransmitter/receiver is placed in the pacemaker canister 164 so that theMRI system 168 can be in operative communication with the pacemakersystem and vice versa. Thus, the pacing system can transmit signals tothe MRI system 168 indicating when the pacemaker is about to deliver apacing pulse to the heart. The transmitted signals may be digital oranalog. In response to this transmitted signal, the MRI system 168 stopsor pauses the MRI switched gradient field (imaging scanning sequence) toallow the pacing pulse to occur. After the pacing pulse has beendelivered to the heart, the MRI system 168 resumes or begins a newimaging scanning sequence.

In another mode of operation, the MRI system 168 sends signals to theimplanted heart pacemaker pulse generator canister 164 through thesensor/transceiver 166 indicating the application of switched gradientfields. The pacemaker may use this information to switch filters orother electronics in and out of the circuit to reduce or eliminatevoltages induced in the pacemaker leads by the gradient fields. Forexample, the pacemaker may switch in additional resistance or inductanceor impedance into the pacing/sensing and/or ground strands based on thesignal from the MRI system 168 signifying the application of thegradient fields.

In another configuration, there is no surface sensor/transceiver orcommunication line to the MRI system 168. Instead, there is a specialsensor in the implanted heart pacemaker pulse generator canister 164that can sense the application of the gradient fields. In responsethereof, the pacemaker switches into the electrical circuit of thepacing/sense and/or ground leads a charging source which is used tocharge the implanted heart pacemaker pulse generator canister 164,leads, and/or electrodes to an electrical potential opposite to thatwhich would be induced by the gradient fields. In this way, the inducedvoltages caused by the gradient fields are cancelled out or reduced to asafe level, by the application of this voltage source.

In a preferred embodiment of the present invention, the charging/voltagesource receives its power from inductively coupling to the MRI system'sradio-frequency field. The oscillating radio-frequency field suppliespower to charge special capacitors in the implanted heart pacemakerpulse generator canister 164. It is noted that other external powersources can be used to power the charging/voltage source in theimplanted heart pacemaker pulse generator canister 164.

FIG. 4 is a diagram of an assembly 170 for the pacemaker generatorcomponents comprising the canister housing 172, a programmable logicunit (PLU) 184, a power source 174, and a pulse generator 176.Additionally, means for communicating with an externalsensor/transceiver is provided by transceiver 180. Other electroniccomponents 178; e.g., signal filters, signal processors, leadconnectors, etc. are also located in the canister 172. The pacing leads182 pass through the canister 172 and connect to the internalelectronics 178. During an NRI examination, the signals transmitted andreceived by the transceiver 180 may be used to synchronize the MRIsystem's scanning sequences with the delivery of the pacing signals.

In another embodiment, as depicted in FIG. 5, the pacing generatorassembly 190 further includes a second power module 186 which may be aninductive coil and/or capacitor bank, suitable for capturing and storingpower from the MRI system's transmitted radio-frequency signal.

In one embodiment, the power stored in the power module 186 is used todevelop an electrical potential in the leads 182 that is opposed to thatwhich is induced by the application of the MRI system's gradient fields.

In another embodiment, the power stored in the power module 186 is usedto operate various switches in the electronics module 178 which mayswitch in or out various power serge protection circuits in-line and/orsignal filters to the leads 182.

In a further embodiment, and referring to FIG. 5, the module 186 may beused to electrically charge the pacemaker canister 172, which is made ofa conductive material, in synchronization with the application of theMRI system's gradient fields so that the electrical potential differencebetween the pacing electrodes and the pacemaker canister 172 is reduced.That is, the sum of the induced voltage difference due to theapplication of the gradient fields plus the voltage difference due tothe application of the electrical charge stored in the power module 186results is a net voltage significantly below any threshold level, abovewhich a problem may develop.

FIG. 6 depicts another assembly 200, which includes the basic componentsof FIG. 5 less the transceiver 180, a gradient field detector 204, and aby-pass switch component 202. By detecting the gradient signal in thepacemaker canister 172 with gradient field detector 204, the pacemakercan switch filters and/or other electronics 178 in or out of thecircuit.

In one embodiment, when no gradient fields are detected, the switch 202is closed to by-pass the electronics component 178, which may be acombination of low-pass, high-pass, notch filters, diodes, and/or otherelectronics. In this mode (switched closed), the pacing pulse (andsensing signals) by-pass the filters components 178. When gradient fielddetector 204 detects the gradient signals, the switch 202 is opened andany gradient fields induced signals in the leads 182 are blocked orsignificantly reduced by the filters components 178. In the open mode,the pacing and sensing signals pass through the filters component 178 aswell.

The gradient detector 204 may communicate the sensing of the gradientfield to other components in the pacemaker via its connection to the PLU184 so that the pacing signal can be modified, if necessary, tocompensate for any distortion it may suffer by now going through thefilters component 178. Additionally, the sensing signal, now alsopassing through the filter components 178 may be distorted. This may becompensated for by including signal recovery/reconstruction logic intothe PLU or into a separate signal-processing component.

Referring back to FIG. 1, by increasing the impedance of the leads 104,the voltage across the tissue gap from the electrodes 112 and thepacemaker canister 102 can be reduced. Inserting a resistor or using ahigher resistive wire for the pacemaker leads 104 will reduce thecurrent induced in the current loop, which includes the virtual loopportion across the (heart 112) tissue to the pacemaker generatorcanister 102.

By using various inductors in-line with the various leads 104, it ispossible to make the leads 104 have a high impedance for the lowfrequency MRI gradient fields frequency and a low impedance for the MRIsystem's radio-frequency frequency. Alternatively, different impedances(inductors/resistors/capacitors) may be switched in-line or out of theleads'circuitry depending on the timing and application of the gradientand/or radio-frequency fields.

In another embodiment, not shown, the pacemakers'electronics can beaugmented to include one or more digital signal processors. Byconverting the sensing signal into a digital signal, the digital signalprocessor (DSP) can reconstruct the sensing signal after it has passedthrough filters and has been distorted by the filtering or otherelements that may have been added to the lead circuit. The DSP may alsobe used to reject any signals that do not have a correct cardiacsignature, thus rejecting any signals caused by the switched gradientfields, which is a non-cardiac signal.

In another embodiment of the present invention, a pacemaker lead orother medical device, having a long conductive lead and functioning inan MRI environment, may be configured, according to the concepts of thepresent invention, to include additional loops to cancel the inducedvoltage effects in the leads of the original current loop formed by theleads.

In FIG. 7, two conductive loops 260 and 270 having the same amount ofarea and in the same plane, positioned in a changing magnetic field 262and 272, develop currents 264 and 274. In FIG. 7, both induced currentsI₁ and I₂ travel in the same direction (clockwise direction shown) atall times as the magnetic field 262 and 272 oscillate.

FIG. 8 shows that by connecting the two conductive loops 260 and 270 ofFIG. 7 to form a single conductor 280, the currents induced in each lobecan be made to cancel each other out. The two loops are connected sothat a single conductor is formed which crosses over itself at 284. Inthis case, as shown in FIG. 8, the two currents 286 and 288 cancel eachother out resulting in net current of zero magnitude around theconductor 280. This type of configuration of conductors in a changingmagnetic field may be used to cancel induced currents in the conductors.

FIG. 9 depicts an implanted pacemaker system 220 comprising a pacinggenerator canister 102, conductive leads 104, and electrodes 112positioned in the heart 106. Additional loops 222 are added to theoverall configuration of the lead 104 in the body with one or morecrossings 224. In accordance with the concepts of the present invention,the plane of the loop 222 is in the same plane as defined by the rest ofthe lead geometry.

The same oscillating magnetic field 110 passes through loop 222 and theloop defined by generator canister 102, conductive leads 104, electrodes112, and conductive path 108 through the body from the heart 106 to thegenerator canister 102. It is noted that the total area enclosed by theloops can be adjusted by adding or removing loops 222 or by changing thearea enclosed by the loops (singly or collectively).

In one embodiment, the total area of the loop 222 is the same as theloop area 226. In another embodiment, the total area of the loop 222 isdifferent from loop area 226. In another embodiment, the plane of loop222 is different from the plane of loop area 226. In yet anotherembodiment, loop 222 and/or loop area 226 do not define a single planebut are curved in three different spatial directions. In yet anotherembodiment, loop 222 consists of at least three loops in threeorthogonal planes.

In a further embodiment, as illustrated in FIG. 11 and will be discussedin more detail below, the new additional loops 222 can be positioned insuch a way as to encircle the pacemaker's generator canister 102. Inanother embodiment, as illustrated in FIG. 10 and will be discussed inmore detail below, the additional loops 222 may be positioned inside thepacemaker's generator canister 102.

Referring back to FIG. 9, a fastener (not shown) can be used at the loopcross over point 224 to allow for adjustment of the loop's enclosed areaand/or orientation and, once adjusted, to lock in the loop'sadjustments. This same fastener can also be used to adjust a pluralityof loops.

In a further embodiment of FIG. 9, pacemaker's generator canister 102may include an orientation subsystem for automatically changing aspatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current. In this embodiment, theorientation subsystem may sense the magnitude of the MRI switchedgradient field induced current (voltage) and spatially tune theorientation of the coils so as to produce more current to oppose the MRIswitched gradient field induced current or less current to oppose theMRI switched gradient field induced current based upon the sensedmagnitude of the MRI switched gradient field induced current (voltage).

In other words, if greater current is needed to oppose the MRI switchedgradient field induced current, the orientation subsystem wouldspatially move or adjust one or more coils such that their surfaceplanes become more perpendicular to the MRI gradient field lines,thereby inducing a greater magnitude of current to oppose the MRIswitched gradient field induced current. On the other hand, if lesscurrent is needed to oppose the MRI switched gradient field inducedcurrent, the orientation subsystem would spatially move or adjust one ormore coils such that their surface planes become less perpendicular andmore parallel to the MRI gradient field lines, thereby inducing a lessermagnitude of current to oppose the MRI switched gradient field inducedcurrent.

In another aspect of the present invention, a selection mechanism can beincluded in the pacemaker system. This selection mechanism is used toadjust the number of loops to include in the circuit.

For example, if the loops are located within the pacemaker canister, theselection mechanism can be used to manually select how many loops toinclude in the lead circuit depending on where the pacemaker can isplaced in the body and the length of the lead. Alternatively, theselection mechanism may be used to automatically select how many loopsto include in the lead circuit depending on where the pacemaker can isplaced in the body and the length of the lead. In this alternativeembodiment, the present invention monitors the voltages on thepacemaker's lead(s) and selects a different number of loops to connectto the lead(s) to cancel any induced voltages. Lastly, the selectionmechanism may be externally programmed and transmitted to thepacemaker's PLU that then monitors and adjusts the number of loops inthe lead circuit.

FIG. 10 is a schematic of a pacemaker system 300 that includes apacemaker canister 302 and the pacemaker's leads 304. The pacemaker'scanister 302 contains a programmable logic unit (PLU) 306, and otherelectronics 310, e.g. a pulse generator, power supply, etc. The system300 further includes conductive loops 308 positioned within thepacemaker canister 302.

The conductive loops are connected to a loop selection component 312that provides means for selectively adjusting the number of loops to beincluded in the leads'circuit 304. The leads 304 are also connected tothe loop selection component 312 so that the leads 304 can beelectrically connected to the loops 308.

The loop selection component 312 connects the loops 308 to theleads'circuit 304 in such a way that any induced voltages in the loops308 caused by changing magnetic fields in the environment, e.g. an MRIenvironment, will cancel out or significantly reduce in magnitude anyinduced voltage along the leads 304 that have also been caused by theenvironment's changing magnetic fields.

In one embodiment, the loop selection component 312 is adjusted manuallyby screws, connection pins, and/or other means.

In another embodiment, the loop selection component 312 is controlled bythe PLU 306. The PLU 306 may include means for receiving loop selectioninstructions from an external transmitter or may include sensors thatmeasure environmental variables, e.g. changing magnetic fields in an MRIenvironment. From this information, the PLU 306 dynamically adjusts theloop selection component's 312 adjustable parameters so as to changewhich loops are included in the leads'circuitry 304. It is noted thatthe loops 308 need not be all in the same plane.

In a further embodiment of FIG. 10, pacemaker's generator canister 302may include an orientation subsystem for automatically changing aspatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current. In this embodiment, theorientation subsystem may sense the magnitude of the MRI switchedgradient field induced current (voltage) and spatially tune theorientation of the coils so as to produce more current to oppose the MRIswitched gradient field induced current or less current to oppose theMRI switched gradient field induced current based upon the sensedmagnitude of the MRI switched gradient field induced current (voltage).

In other words, if greater current is needed to oppose the MRI switchedgradient field induced current, the orientation subsystem wouldspatially move or adjust one or more coils such that their surfaceplanes become more perpendicular to the MRI gradient field lines,thereby inducing a greater magnitude of current to oppose the MRIswitched gradient field induced current. On the other hand, if lesscurrent is needed to oppose the MRI switched gradient field inducedcurrent, the orientation subsystem would spatially move or adjust one ormore coils such that their surface planes become less perpendicular andmore parallel to the MRI gradient field lines, thereby inducing a lessermagnitude of current to oppose the MRI switched gradient field inducedcurrent.

FIG. 11 is a schematic of another pacemaker system 320. Pacemaker system320 includes conductive loops 322 positioned externally to a pacemakercanister 302. In this embodiment, the loops 332 are connected to aninput port connection 330 and to an output port connection 334 which areelectrically connected to the loop selection component 324 locatedinside the pacemaker canister 302. Additionally, the pacemaker leads 304are connected to an electrical connector 332 that is electricallyconnected to the loop selection component 324. It is noted that theconductive loops 322 need not be all in the same plane.

In a further embodiment of FIG. 11, pacemaker system 320 may include anorientation subsystem for automatically changing a spatial orientationof the coil to modify the strength of the MRI switched gradient fieldinduced current. In this embodiment, the orientation subsystem may sensethe magnitude of the MRI switched gradient field induced current(voltage) and spatially tune the orientation of the coils so as toproduce more current to oppose the MRI switched gradient field inducedcurrent or less current to oppose the MRI switched gradient fieldinduced current based upon the sensed magnitude of the MRI switchedgradient field induced current (voltage).

In other words, if greater current is needed to oppose the MRI switchedgradient field induced current, the orientation subsystem wouldspatially move or adjust one or more coils such that their surfaceplanes become more perpendicular to the MRI gradient field lines,thereby inducing a greater magnitude of current to oppose the MRIswitched gradient field induced current. On the other hand, if lesscurrent is needed to oppose the MRI switched gradient field inducedcurrent, the orientation subsystem would spatially move or adjust one ormore coils such that their surface planes become less perpendicular andmore parallel to the MRI gradient field lines, thereby inducing a lessermagnitude of current to oppose the MRI switched gradient field inducedcurrent.

FIG. 12 depicts a medical procedure in which a catheter 406 or othermedical device, e.g. a guidewire, which is comprised of conductive leadsor other conductive components, may be partially inserted into a body402 and partially external to the body. In an MRI environment, suchconductive medical devices 406 can develop problems like heating,induced voltages, etc. caused by the changing magnetic fields of the MRIsystem. To compensate for induced currents and/or induced voltages insuch devices 406, a voltage compensation unit (VCU) 410 is electricallyconnected to the medical device 406 via conductive leads 412 andelectrical connectors 414, externally to the patient's body 402.

The medical device 406 is constructed with additional electricalconnectors 414 to allow for easy attachment of the VCU device 410. TheVCU device 410 is connected to a power supply or may have a built inpower supply, e.g. batteries. The VCU device 410 has sensors built intoit, which monitor the voltages of the conductive components in themedical device 406, and delivers opposing voltages to the medical device406 to cancel out or significantly reduce any induced voltages caused bythe changing magnetic fields in an MRI (or other) environment.

Additionally or alternatively, the VCU device 410 has sensors to detectthe changing magnetic fields of the MRI system and can synchronize theapplication of the canceling voltage with the MRI System's changingfields.

In another embodiment depicted in FIG. 13, the VCU device 420 isconnected to the MRI system 422 via communication means 424 so that thestart and end of the application of the MRI system's 422 fields may becommunicated to the VCU device 420. Other information that may berequired (field strengths to be applied, MRI scan sequence, etc.) mayalso be communicated to the VCU device 420. The communication means 424may be electrical wires/coaxial/shielded/other, optical fiber, or aradio-frequency transmitter/receiver, or some sonic means ofcommunication.

The conductive lead of a heart pacemaker is a filer winding. The filerwinding may consist of two or more conductive stands coiled together ina spring-like configuration. The current (pulses, signals) then flowsover the surface and through the contact points between one loop and theadjacent loop of the winding, rather than following the windings of theindividual conductive strands. This occurs because there is nosignificant insulating material or surface coating between the contactpoints of the windings.

In accordance with the present invention, to reduce the alternating,induced current flowing, caused by a magnetic resonance system'schanging magnetic fields, through the, for example, pacemaker's windingleads, the inductance value of the pacemaker's lead may be changed toincrease the overall impedance of the pacemaker's lead.

Thus in one embodiment, a suitable radio-frequency choke is insertedinline with the pacemaker's lead, preferable near the distal tip. Forexample, referring back to FIG. 2, and to the embodiment therein,electronic component 146 and/or 144 may comprise a radio-frequencychoke. In a preferred embodiment, the radio-frequency choke has aninductance value of about 10 microHenries. In another embodiment, theinductance value is about 2 microHenries.

The specific value of inductance to introduce into the, for example,pacemaker's lead depends in part on the frequency of the induced signalfrom the MRI system's imaging sequence that is to be blocked orsignificantly reduced.

FIG. 14 shows a portion of a coiled multi-filer lead 450. As illustratedin FIG. 14, lead 450 includes a plurality of coil loops 452; each coilloop 452 consists of three conductive strands 454, 456, and 458. Acurrent 460 through the lead 450 can cross contact points 464, 466, and462 between the strands as well as the coil contact points 468 and 470.Thus, the current 460 does not follow the coiling of the lead'sconductive strands 454, 456, and 458.

FIG. 15 shows a portion of a coiled lead assembly 480 including a region482 that has an insulating coating 484 applied to its surface. Thecoiled lead assembly 480 is depicted in an elongated position in whichadjacent coil windings are not in contact with one another. It is to beunderstood that the normal, relaxed position of the lead assembly 480has all adjacent coiled windings in contact.

With the addition of an insulated coating 484 over the winding region482, the current 490, 492, and 494 is now forced to substantially followthe curvature of the coiled winding 482, thus forming an inductive coilinline with the conductive lead regions 486 and 488 which do not have aninsulated coating. The inductive value of the created inductor can beadjusted by adjusting the length of the region to which the insulativecoating 484 is applied.

It is noted that the coating 484 may be a partially resistive material.In such an example, the inductance is then adjusted by adjusting theresistive properties of the material 484.

FIG. 16 is a schematic of a coiled lead assembly 500 comprised ofuninsulated regions 502, 504, and 506, and coated insulated regions 508and 510 with coatings 512, and 514, respectively. Through theapplication of the coating, the current is forced to substantiallyfollow the curvature of the coiled windings, thus forming an inductivecoil inline with the conductive lead regions that do not have a coatingapplied thereto. The inductive value of the created inductor can beadjusted by adjusting the length of the region to which the insulativecoatings 512 and 514 are applied. In one embodiment, coatings 512 and514 are the same coatings. In another embodiment, the coatings 512 and514 are different materials.

It is noted that coatings 512 and 514 may be the same coating materialbut having differing properties, e.g., the thickness of the coatings, orthe length of the coated region 508 and 510. It is further noted thatthe two-coated regions 508 and 510 may have different inductive values.It is also noted that more than two different regions along the lengthof the lead assembly can be coated.

FIG. 17 is a schematic of a portion of a coiled lead assembly 520including at least one region 524 with a coating applied thereto.Through the application of the coating, the current is forced tosubstantially follow the curvature of the coiled windings, thus formingan inductive coil inline with the conductive lead regions 522 and 526that do not have a coating applied thereto. The inductive value of thecreated inductor can be adjusted by adjusting the length of the regionto which the insulative coating 524 is applied. Additionally, throughthe coated region 524 is positioned a rod 528 which also changes theinductive value of the coated region 524. It is noted that the rod 528may be of ferrite material. It is further noted that multiple rods canbe inserted into multiple coated regions along the length of the coiledlead.

It is noted that multiple coatings can be applied to the same coatedregion of the coiled lead wherein the multiple coating layers may becomprised of different materials. It is further noted that one or morelayers of the multiple layers of coatings may comprise ferrite material.

In another embodiment of the present invention, the heating and/orinduced voltages on catheters or guide wires is controlled orsubstantially eliminated by introducing or creating detunedcharacteristic impedance at a proximal ends (ends that are not withinthe body) of the catheters or guide wires. This introduction or creationof detuned characteristic impedance will be discussed in more detailwith respect to FIGS. 18-21.

As noted above, during MRI procedures, catheters and guide wires (wirelines), with or without grounded shielding, are used to measurephysiological signals. In such instances, two-wire catheters or guidewires having a grounded shield have one conductor that carries theactual measured signal and the other wire is grounded. In terms ofcharacteristic impedance, the two-wire catheters or guide wires having agrounded shield are unbalanced. In contrast, a single wire catheter orguide wire has characteristic impedance that is balanced.

According to the concepts of the present invention, the characteristicimpedance of the catheters and guide wires, used during MRI procedures,should be unbalanced at the proximal end, under all conditions, toreduce or eliminate heating and induced voltages. To realize thisreduction or elimination of heating and induced voltages at the proximalend of the catheters and guide wires, used during MRI procedures, bycreating an unbalanced characteristic impedance, the present inventionproposes providing a Balun along the catheter and/or guide wire or atthe proximal end of the catheter and/or guide wire.

Using a Balun to maintain unbalanced characteristic impedance, thereactance at the distal end of the catheter and/or guide wire approachesinfinity. Thus, even when there is some potential on the wire, theunbalanced characteristic impedance has approximately four times theground loop looses of a balanced line, thereby substantially avoidingany incident of thermal injury. An example of such an arrangement isillustrated in FIG. 18.

As illustrated in FIG. 18, a guide wire or catheter 650 hascharacteristic impedance due to its intrinsic resistance from intrinsicresistor capacitors R_(P) and its intrinsic inductance from intrinsicinductor L. To create the unbalanced characteristic impedance at theproximal end of the guide wire or catheter 650, a Balun 600 is placedalong the guide wire or catheter 650. In other words, the Balun 600 isin vitro.

The Balun 600 includes a variable capacitor C₁ connected in parallelwith the guide wire or catheter 650 and two variable capacitors C₂ andC₃ connected in series with the guide wire or catheter 650. It is notedthat one end of the variable capacitor C₂ is connected to the shield 625and ground or a known voltage. The capacitance of the variablecapacitors C₁, C₂, and C₃ are adjusted to create the unbalancedcharacteristic impedance.

More specifically, the variable capacitors C₁, C₂, and C₃ may be usedfor both matching and providing a certain amount of balancing for theguide wire or catheter 650 characteristic impedance. In this example,the variable capacitors C₁, C₂, and C₃ lift the voltage on the guidewire or catheter 650 from ground. The larger the reactance of thevariable capacitors C₁, C₂, and C₃, the more symmetric and balanced thecircuit of the guide wire or catheter 650 becomes. Conversely, accordingto the concepts of the present invention, if the reactive capacitance ofthe Balun 600 is detuned (made less resonant), the circuit of the guidewire or catheter 650 becomes asymmetric and unbalanced, breaking down,to reduce the chances of thermal injury at the distal end of the guidewire or catheter 650 due to heating from induced voltages.

FIG. 19 illustrates another embodiment of the present invention whereina guide wire or catheter 6500 has characteristic impedance due to itsintrinsic capacitance from intrinsic capacitors C_(t), and C_(s) and itsintrinsic inductance from intrinsic inductor L. To create the unbalancedcharacteristic impedance at the proximal end of the guide wire orcatheter 6500, a Balun 6000 is connected across the proximal end of theguide wire or catheter 6500. In other words, the Balun 6000 is outsidethe body at the proximal end of the guide wire or catheter 650. Byhaving the Balun 6000 outside the body, the varying of the reactance ofthe guide wire or catheter 6500 can be readily and manually controlled.

The Balun 6000 includes a variable capacitor C₁ connected in parallelwith the guide wire or catheter 6500 and a variable capacitor C₂connected in series with the guide wire or catheter 6500. It is notedthat one end of the variable capacitor C₁ is connected to the shield6250 and ground or a known voltage. The capacitance of the variablecapacitors C₁ and C₂ are adjusted to create the unbalancedcharacteristic impedance.

More specifically, the variable capacitors C₁, and C₂ may be used forboth matching and providing a certain amount of balancing for the guidewire or catheter 6500 characteristic impedance. In this example, thevariable capacitors C₁, C₂, and C₃ lift the voltage on the guide wire orcatheter 6500 from ground. The larger the reactance of the variablecapacitors C₁ and C₂, the more symmetric and balanced the circuit of theguide wire or catheter 6500 becomes. Conversely, according to theconcepts of the present invention, if the reactive capacitance of theBalun 6000 is detuned (made less resonant), the circuit of the guidewire or catheter 6500 becomes asymmetric and unbalanced, breaking down,to reduce the chances of thermal injury at the distal end of the guidewire or catheter 6500 due to heating from induced voltages.

FIG. 20 illustrates a further embodiment of the present inventionwherein a guide wire or catheter 800 is connected to a Balun 700. TheBalun 700 includes a variable capacitor 710, a copper foil 720, and anon-conductive tuning bolt 730. The Balun 700 is further connected tothe output of the probe 800

The Balun 700 adjusts its characteristic impedance by increasing ordecreasing the number wire coils are found within the copper foil 720.The combination of the coils and the copper foil 720 forms a variablecapacitor, having it impedance determined by the change in the surfacearea of the coils positioned opposite of the copper foil 720. As morecoils are introduced into the volume created by the copper foil 720, thecapacitance of this combination increases. Moreover, as fewer coils areintroduced into the volume created by the copper foil 720, thecapacitance of this combination decreases. Thus, the capacitance of theBalun 700 is adjusted to create the unbalanced characteristic impedance.

FIG. 21 illustrates another embodiment of the present invention whereina guide wire or catheter 900 is electronically isolated by a voltagecontrol unit to always appear as an unbalanced line to any possiblemagnetic field that may be applied from a magnetic resonance imager unit(not shown). As current begins to flow due to the changing magneticfields from the magnetic resonance imaging, a tapped voltage from avoltage-controlled oscillator in the magnetic resonance imaging unit isapplied across terminals X1 and X2 of the voltage control unit.

According to the concepts of the present invention, to automaticallymaintain an unbalanced characteristic impedance at the distal end of theguide wire or catheter 900, a capacitance unbalanced balun unit 7000,located within the voltage control unit, is connected through a variableinductor 910 to the proximal end of the guide wire or catheter 900. Inother words, the voltage control unit containing the capacitanceunbalanced balun unit 7000 is outside the body at the proximal end ofthe guide wire or catheter 900. By having the capacitance unbalancedbalun unit 7000 and variable. inductor 910 outside the body, the varyingof the reactance (X0) of the guide wire or catheter 900 can be readilyadjusted and automatically controlled by the voltage control unitcircuit's reactance to the tapped voltage from the voltage-controlledoscillator in the magnetic resonance imaging unit as it is appliedacross X1 and X2 for any instance of time from time zero (T0) orinstantiation of the magnetic resonance imaging radio-frequency pulses.

The capacitance unbalanced balun unit 7000 includes two non-magnetictrimmer capacitors C1 and C2 connected in parallel with LC circuits(L1,C3) and (L2,C4), respectively, setting up a simplified dual Tnetwork that is effectively in series with the guide wire or catheter900. It is noted that one end of the simplified dual T network isconnected to neutral H1 and the other end is connected to a continuouslyvariable voltage H2, based on inputs to the circuit from thevoltage-controlled oscillator in the magnetic resonance imaging unit atX1 and X2. The reactance (X0) of the LC circuits in the T network isautomatically adjusted to create the desired unbalanced characteristicimpedance.

More specifically, the T network L1, C1, C3 and L2, C2, C4 respectively,may be used for both matching and unmatching characteristic impedance ofthe guide wire or catheter 900 and to provide a certain amount ofbalancing or unbalancing for the guide wire or catheter 900 by varyingthe circuit's capacitive or inductive reactance (X0).

In this example, as the voltage from the voltage-controlled oscillatorin the magnetic resonance imaging unit is provided to the voltagecontrol unit (X1 X2), the two non-magnetic trimmer capacitors C1 and C2,connected in parallel with LC circuits, (L1,C3) and (L2,C4), lift thevoltage on the guide wire or catheter 900 from ground to an unbalancedstate with respect to the radio-frequency pulse applied by the magneticresonance imaging unit. The reactance of the T network and its LCcircuits, (L1,C3) and (L2,C4), respectively, cause the guide wire orcatheter 900 to become asymmetric and unbalanced, automatically breakingdown the reactance to ensure that resonance for the guide wire orcatheter 900 is never present, thus reducing the chances of thermalinjury at the distal end of the guide wire or catheter 900 due toheating from induced voltages.

In summary, the present invention is directed to a system for reducingthe effects of MRI induced signals to a safe level having a medicaldevice that includes a housing having electronic circuitry therein, afirst lead to provide an electrical path for a stimulation signalgenerated by the electronic circuitry to be applied to a desired tissueregion, a second lead to provide an electrical path for a sensedphysiological condition of the desired tissue region to be communicatedto the electronic circuitry, and a third lead to provide an electricalground. A diode is connected to the first lead to significantly reduceMRI induced signals from traveling along the first lead to theelectronic circuitry.

This embodiment of the present invention may also include a filter,connected to the second lead, to significantly reduce MRI inducedsignals from traveling along the second lead to the electroniccircuitry; a second filter, connected to the third lead, tosignificantly reduce MRI induced signals from traveling along the thirdlead to the electronic circuitry; a sensor to sense application ofswitched MRI gradient fields; an electronic component, connected to thesecond lead, to significantly reduce MRI induced signals from travelingalong the second lead to the electronic circuitry; and/or a switch,connected to the sensor and the electronic component, to operativelyconnect the electronic component to the second lead when the sensorsenses the application of switched MRI gradient fields and tooperatively disconnect the electronic component from the second leadwhen the sensor fails to sense the application of switched MRI gradientfields.

In this embodiment the electronic component may be a filter, impedance,an inductor, a resistor, and/or a capacitor. The electronic componentmay be a source that generates an electrical potential opposite to thatwhich would be induced by the MRI switched gradient fields so as toreduce voltages induced by the MRI switched gradient fields to a safelevel or a coil that generates a MRI switched gradient field inducedcurrent opposite to that which would be induced by the MRI switchedgradient fields in the leads so as to reduce voltages induced by the MRIswitched gradient fields to a safe level wherein the coil may be curvedin three different spatial directions. The electronic component havingthe coil may also include an orientation subsystem for changing aspatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

The electronic component may include a plurality of coils, each coilgenerating a MRI switched gradient field induced current opposite tothat which would be induced by the MRI switched gradient fields in theleads so as to reduce voltages induced by the MRI switched gradientfields to a safe level, and a switch connecting independently a numberof the plurality of the coils, the number of connected coilscorresponding to an amount of the voltage induced by the MRI switchedgradient fields and a current level produced in each coil when thesensor senses the application of switched MRI gradient fields whereinthe coils are curved in three different spatial directions. Theelectronic component may also include an orientation subsystem forchanging a spatial orientation of the coils to modify the strength ofthe MRI switched gradient field induced currents.

The electronic component may be three orthogonally planar coils, eachcoil generating a MRI switched gradient field induced current oppositeto that which would be induced by the MRI switched gradient fields inthe leads so as to reduce voltages induced by the MRI switched gradientfields to a safe level, and a switch connecting independently a numberof the coils, the number of connected coils corresponding to an amountof the voltage induced by the MRI switched gradient fields and a currentlevel produced in each coil when the sensor senses the application ofswitched MRI gradient fields. The electronic component may also includean orientation subsystem for changing a spatial orientation of the coilsto modify the strength of the MRI switched gradient field inducedcurrents wherein each coil defines a distinct plane that transverses theMRI switched gradient fields.

This embodiment may also include a receiver to receive a signal from aMRI system indicating an application of switched MRI gradient fields; anelectronic component, operatively connected to the second lead, tosignificantly reduce MRI induced signals from traveling along the secondlead to the electronic circuitry; and a switch, operatively connected tothe receiver and the electronic component, to operatively connect theelectronic component to the second lead when the receiver receives anindication of the application of switched MRI gradient fields and tooperatively disconnect the electronic component from the second leadwhen the receiver receives a signal indicating no application ofswitched MRI gradient fields. The electronic component may be a filter,impedance, an inductor, a resistor, and/or a capacitor.

The electronic component may be a source that generates an electricalpotential opposite to that which would be induced by the MRI switchedgradient fields so as to reduce voltages induced by the MRI switchedgradient fields to a safe level or a coil that generates a MRI switchedgradient field induced current opposite to that which would be inducedby the MRI switched gradient fields in the leads so as to reducevoltages induced by the MRI switched gradient fields to a safe levelwherein the coil may be curved in three different spatial directions.The electronic component having the coil may also include an orientationsubsystem for changing a spatial orientation of the coil to modify thestrength of the MRI switched gradient field induced current.

The electronic component may include a plurality of coils, each coilgenerating a MRI switched gradient field induced current opposite tothat which would be induced by the MRI switched gradient fields in theleads so as to reduce voltages induced by the MRI switched gradientfields to a safe level, and a switch connecting independently a numberof the plurality of the coils, the number of connected coilscorresponding to an amount of the voltage induced by the MRI switchedgradient fields and a current level produced in each coil when thesensor senses the application of switched MRI gradient fields whereinthe coils are curved in three different spatial directions. Theelectronic component may also include an orientation subsystem forchanging a spatial orientation of the coils to modify the strength ofthe MRI switched gradient field induced currents.

The electronic component may be three orthogonally planar coils, eachcoil generating a MRI switched gradient field induced current oppositeto that which would be induced by the MRI switched gradient fields inthe leads so as to reduce voltages induced by the MRI switched gradientfields to a safe level, and a switch connecting independently a numberof the coils, the number of connected coils corresponding to an amountof the voltage induced by the MRI switched gradient fields and a currentlevel produced in each coil when the sensor senses the application ofswitched MRI gradient fields. The electronic component may also includean orientation subsystem for changing a spatial orientation of the coilsto modify the strength of the MRI switched gradient field inducedcurrents wherein each coil defines a distinct plane that transverses theMRI switched gradient fields.

In another embodiment, the present invention is directed to a system forreducing the effects of MRI induced signals to a safe level. The systemincludes a MRI system; a medical device; and a transceiver to providecommunication between the MRI system and the medical device. The medicaldevice has a housing having electronic circuitry therein, abi-directional lead to provide an electrical path for a stimulationsignal generated by the electronic circuitry to be applied to a desiredtissue region and to provide an electrical path for a sensedphysiological condition of the desired tissue region to be communicatedto the electronic circuitry, and a lead to provide an electrical ground.The medical device indicates to the MRI system, through the transceiver,when the stimulation signal will be applied to the desired tissueregion. The MRI system, in response to the indication from the medicaldevice of when the stimulation signal will be applied to the desiredtissue region, terminates a production of MRI switched gradient fields.

This embodiment may further include a filter, connected to thebi-directional lead, to significantly reduce MRI induced signals fromtraveling along the bi-directional lead to the electronic circuitry; afilter, connected to the lead, to significantly reduce MRI inducedsignals from traveling along the lead to the electronic circuitry; asensor to sense application of switched MRI gradient fields; anelectronic component, connected to the bi-directional lead, tosignificantly reduce MRI induced signals from traveling along thebi-directional lead to the electronic circuitry; and/or a switch,connected to the sensor and the electronic component, to operativelyconnect the electronic component to the bi-directional lead when thesensor senses the application of switched MRI gradient fields and tooperatively disconnect the electronic component from the bi-directionallead when the sensor fails to sense the application of switched MRIgradient fields. The electronic component may be a filter, impedance, aninductor, a resistor, and/or a capacitor.

The electronic component may be a source that generates an electricalpotential opposite to that which would be induced by the MRI switchedgradient fields so as to reduce voltages induced by the MRI switchedgradient fields to a safe level or a coil that generates a MRI switchedgradient field induced current opposite to that which would be inducedby the MRI switched gradient fields in the leads so as to reducevoltages induced by the MRI switched gradient fields to a safe levelwherein the coil may be curved in three different spatial directions.The electronic component having the coil may also include an orientationsubsystem for changing a spatial orientation of the coil to modify thestrength of the MRI switched gradient field induced current.

The electronic component may include a plurality of coils, each coilgenerating a MRI switched gradient field induced current opposite tothat which would be induced by the MRI switched gradient fields in theleads so as to reduce voltages induced by the MRI switched gradientfields to a safe level, and a switch connecting independently a numberof the plurality of the coils, the number of connected coilscorresponding to an amount of the voltage induced by the MRI switchedgradient fields and a current level produced in each coil when thesensor senses the application of switched MRI gradient fields whereinthe coils are curved in three different spatial directions. Theelectronic component may also include an orientation subsystem forchanging a spatial orientation of the coils to modify the strength ofthe MRI switched gradient field induced currents.

The electronic component may be three orthogonally planar coils, eachcoil generating a MRI switched gradient field induced current oppositeto that which would be induced by the MRI switched gradient fields inthe leads so as to reduce voltages induced by the MRI switched gradientfields to a safe level, and a switch connecting independently a numberof the coils, the number of connected coils corresponding to an amountof the voltage induced by the MRI switched gradient fields and a currentlevel produced in each coil when the sensor senses the application ofswitched MRI gradient fields. The electronic component may also includean orientation subsystem for changing a spatial orientation of the coilsto modify the strength of the MRI switched gradient field inducedcurrents wherein each coil defines a distinct plane that transverses theMRI switched gradient fields.

In a further embodiment, the present invention is directed to a systemfor reducing the effects of MRI induced signals to a safe level. Thesystem includes a medical device wherein the medical device has ahousing having electronic circuitry therein, leads to provide anelectrical path for a stimulation signal generated by the electroniccircuitry to be applied to a desired tissue region and to provide anelectrical path for a sensed physiological condition of the desiredtissue region to be communicated to the electronic circuitry, a sensorto sense application of switched MRI gradient fields, an electroniccomponent, operatively connected to the leads, to significantly reduceMRI induced signals from traveling along the leads to the electroniccircuitry, and a switch, operatively connected to the sensor and theelectronic component, to operatively connect the electronic component tothe leads when the sensor senses the application of switched MRIgradient fields and to operatively disconnect the electronic componentfrom the leads when the sensor fails to sense the application ofswitched MRI gradient fields.

In this embodiment, the electronic component may be a source thatgenerates an electrical potential opposite to that which would beinduced by the MRI switched gradient fields so as to reduce voltagesinduced by the MRI switched gradient fields to a safe level or a coilthat generates a MRI switched gradient field induced current opposite tothat which would be induced by the MRI switched gradient fields in theleads so as to reduce voltages induced by the MRI switched gradientfields to a safe level wherein the coil may be curved in three differentspatial directions. The electronic component having the coil may alsoinclude an orientation subsystem for changing a spatial orientation ofthe coil to modify the strength of the MRI switched gradient fieldinduced current.

The electronic component may include a plurality of coils, each coilgenerating a MRI switched gradient field induced current opposite tothat which would be induced by the MRI switched gradient fields in theleads so as to reduce voltages induced by the MRI switched gradientfields to a safe level, and a switch connecting independently a numberof the plurality of the coils, the number of connected coilscorresponding to an amount of the voltage induced by the MRI switchedgradient fields and a current level produced in each coil when thesensor senses the application of switched MRI gradient fields whereinthe coils are curved in three different spatial directions. Theelectronic component may also include an orientation subsystem forchanging a spatial orientation of the coils to modify the strength ofthe MRI switched gradient field induced currents.

The electronic component may be three orthogonally planar coils, eachcoil generating a MRI switched gradient field induced current oppositeto that which would be induced by the MRI switched gradient fields inthe leads so as to reduce voltages induced by the MRI switched gradientfields to a safe level, and a switch connecting independently a numberof the coils, the number of connected coils corresponding to an amountof the voltage induced by the MRI switched gradient fields and a currentlevel produced in each coil when the sensor senses the application ofswitched MRI gradient fields. The electronic component may also includean orientation subsystem for changing a spatial orientation of the coilsto modify the strength of the MRI switched gradient field inducedcurrents wherein each coil defines a distinct plane that transverses theMRI switched gradient fields.

In a further embodiment, the present invention is directed to anelectrical lead component for a medical device that reduces the effectsof MRI induced signals to a safe level. The electrical lead componentincludes a medical device electrical lead capable of providing anelectrical path to a desired tissue region and a coil that generates aMRI switched gradient field induced current opposite to that which wouldbe induced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level. The coil may be curved in threedifferent spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to anelectrical lead component for a medical device that reduces the effectsof MRI induced signals to a safe level. The electrical lead componentincludes a medical device electrical lead capable of providing anelectrical path to a desired tissue region and a plurality of coils,each coil generating a MRI switched gradient field induced current sucha combination of the MRI switched gradient field induced currentsprovide a combined current that is opposite to that which would beinduced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level. The coil may be curved in threedifferent spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to anelectrical lead component for a medical device that reduces the effectsof MRI induced signals to a safe level. The electrical lead componentincludes a medical device electrical lead capable of providing anelectrical path to a desired tissue region and three orthogonally planarcoils, each coil generating a MRI switched gradient field inducedcurrent such a combination of the MRI switched gradient field inducedcurrents provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

In a further embodiment, the present invention is directed to anelectrical lead component for a medical device that reduces the effectsof MRI induced signals to a safe level. The electrical lead componentincludes a medical device electrical lead capable of providing anelectrical path to a desired tissue region; a plurality of coils, eachcoil generating a MRI switched gradient field induced current; a sensorto measure a strength of voltages induced by the MRI switched gradientfields; and a switching device, operatively connected to the sensor andplurality of coils, to operatively connect a number of the plurality ofcoils in response to the measured strength of voltages induced by theMRI switched gradient fields such that a combination of the MRI switchedgradient field induced currents produced by the number of operativelyconnected switches provide a combined current that is opposite to thatwhich would be induced by the MRI switched gradient fields in themedical device electrical lead so as to reduce voltages induced by theMRI switched gradient fields to a safe level. The coils may be curved inthree different spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to anelectrical lead component for a medical device that reduces the effectsof MRI induced signals to a safe level. The electrical lead componentincludes a medical device electrical lead capable of providing anelectrical path to a desired tissue region; three orthogonally planarcoils, each coil generating a MRI switched gradient field inducedcurrent; a sensor to measure a strength of voltages induced by the MRIswitched gradient fields; and a switching device, operatively connectedto the sensor and the coils, to operatively connect a number of thecoils in response to the measured strength of voltages induced by theMRI switched gradient fields such that a combination of the MRI switchedgradient field induced currents produced by the number of operativelyconnected switches provide a combined current that is opposite to thatwhich would be induced by the MRI switched gradient fields in themedical device electrical lead so as to reduce voltages induced by theMRI switched gradient fields to a safe level.

In a further embodiment, the present invention is directed to anelectrical lead component for a medical device that reduces the effectsof MRI induced signals to a safe level. The electrical lead componentincludes a medical device electrical lead capable of providing anelectrical path to a desired tissue region; a plurality of coils, eachcoil generating a MRI switched gradient field induced current; atransceiver to receive a signal indicating a number of coils to beconnected; and a switching device, operatively connected to thetransceiver and plurality of coils, to operatively connect a number ofthe plurality of coils in response to the received signal indicating thenumber of coils to be connected such that a combination of the MRIswitched gradient field induced currents produced by the number ofoperatively connected switches provide a combined current that isopposite to that which would be induced by the MRI switched gradientfields in the medical device electrical lead so as to reduce voltagesinduced by the MRI switched gradient fields to a safe level. The coilsmay be curved in three different spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to anelectrical lead component for a medical device that reduces the effectsof MRI induced signals to a safe level. The electrical lead componentincludes a medical device electrical lead capable of providing anelectrical path to a desired tissue region; three orthogonally planarcoils, each coil generating a MRI switched gradient field inducedcurrent; a transceiver to receive a signal indicating a number of coilsto be connected; and a switching device, operatively connected to thetransceiver and the coils, to operatively connect a number of the coilsin response to the received signal indicating the number of coils to beconnected such that a combination of the MRI switched gradient fieldinduced currents produced by the number of operatively connectedswitches provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

In a further embodiment, the present invention is directed to a medicaldevice that reduces the effects of MRI induced signals to a safe level.The medical device includes a medical device capable of providingmedical treatment to a desired tissue region and a coil that generates aMRI switched gradient field induced current opposite to that which wouldbe induced by the MRI switched gradient fields in the medical device soas to reduce voltages induced by the MRI switched gradient fields to asafe level. The coil may be curved in three different spatialdirections.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to a medicaldevice that reduces the effects of MRI induced signals to a safe level.The medical device includes a medical device capable of providingmedical treatment to a desired tissue region and a plurality of coils,each coil generating a MRI switched gradient field induced current sucha combination of the MRI switched gradient field induced currentsprovide a combined current that is opposite to that which would beinduced by the MRI switched gradient fields in the medical device so asto reduce voltages induced by the MRI switched gradient fields to a safelevel. The coils may be curved in three different spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to a medicaldevice that reduces the effects of MRI induced signals to a safe level.The medical device includes a medical device capable of providingmedical treatment to a desired tissue region and three orthogonallyplanar coils, each coil generating a MRI switched gradient field inducedcurrent such a combination of the MRI switched gradient field inducedcurrents provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical device soas to reduce voltages induced by the MRI switched gradient fields to asafe level.

In a further embodiment, the present invention is directed to a medicaldevice that reduces the effects of MRI induced signals to a safe level.The medical device includes a medical device capable of providingmedical treatment to a desired tissue region; a plurality of coils, eachcoil generating a MRI switched gradient field induced current; a sensorto measure a strength of voltages induced by the MRI switched gradientfields; and a switching device, operatively connected to the sensor andplurality of coils, to operatively connect a number of the plurality ofcoils in response to the measured strength of voltages induced by theMRI switched gradient fields such that a combination of the MRI switchedgradient field induced currents produced by the number of operativelyconnected switches provide a combined current that is opposite to thatwhich would be induced by the MRI switched gradient fields in themedical device so as to reduce voltages induced by the MRI switchedgradient fields to a safe level. The coils may be curved in threedifferent spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to a medicaldevice that reduces the effects of MRI induced signals to a safe level.The medical device includes a medical device capable of providingmedical treatment to a desired tissue region; three orthogonally planarcoil, each coil generating a MRI switched gradient field inducedcurrent; a sensor to measure a strength of voltages induced by the MRIswitched gradient fields; and a switching device, operatively connectedto the sensor and plurality of coils, to operatively connect a number ofthe plurality of coils in response to the measured strength of voltagesinduced by the MRI switched gradient fields such that a combination ofthe MRI switched gradient field induced currents produced by the numberof operatively connected switches provide a combined current that isopposite to that which would be induced by the MRI switched gradientfields in the medical device so as to reduce voltages induced by the MRIswitched gradient fields to a safe level.

In a further embodiment, the present invention is directed to a medicaldevice that reduces the effects of MRI induced signals to a safe level.The medical device includes a medical device capable of providingmedical treatment to a desired tissue region; a plurality of coils, eachcoil generating a MRI switched gradient field induced current; atransceiver to receive a signal indicating a number of coils to beconnected; and a switching device, operatively connected to thetransceiver and the coils, to operatively connect a number of the coilsin response to the received signal indicating the number of coils to beconnected such that a combination of the MRI switched gradient fieldinduced currents produced by the number of operatively connectedswitches provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level. The coils may be curved in threedifferent spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to a medicaldevice that reduces the effects of MRI induced signals to a safe level.The medical device includes a medical device capable of providingmedical treatment to a desired tissue region; three orthogonally planarcoil, each coil generating a MRI switched gradient field inducedcurrent; a transceiver to receive a signal indicating a number of coilsto be connected; and a switching device, operatively connected to thetransceiver and the coils, to operatively connect a number of the coilsin response to the received signal indicating the number of coils to beconnected such that a combination of the MRI switched gradient fieldinduced currents produced by the number of operatively connectedswitches provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

In a further embodiment, the present invention is directed to a voltagecontrol unit that reduces the effects of MRI induced signals upon amedical device to a safe level. The voltage control unit includes a coilthat generates a MRI switched gradient field induced current opposite tothat which would be induced by the MRI switched gradient fields in themedical device so as to reduce voltages induced by the MRI switchedgradient fields to a safe level. The coil may be curved in threedifferent spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to a voltagecontrol unit that reduces the effects of MRI induced signals upon amedical device to a safe level. The voltage control unit includes aplurality of coils, each coil generating a MRI switched gradient fieldinduced current such a combination of the MRI switched gradient fieldinduced currents provide a combined current that is opposite to thatwhich would be induced by the MRI switched gradient fields in themedical device so as to reduce voltages induced by the MRI switchedgradient fields to a safe level. The coils may be curved in threedifferent spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to a voltagecontrol unit that reduces the effects of MRI induced signals upon amedical device to a safe level. The voltage control unit includes threeorthogonally planar coils, each coil generating a MRI switched gradientfield induced current such a combination of the MRI switched gradientfield induced currents provide a combined current that is opposite tothat which would be induced by the MRI switched gradient fields in themedical device so as to reduce voltages induced by the MRI switchedgradient fields to a safe level.

In a further embodiment, the present invention is directed to a voltagecontrol unit that reduces the effects of MRI induced signals upon amedical device to a safe level. The voltage control unit includes aplurality of coils, each coil generating a MRI switched gradient fieldinduced current; a sensor to measure a strength of voltages induced bythe MRI switched gradient fields; and a switching device, operativelyconnected to the sensor and plurality of coils, to operatively connect anumber of the plurality of coils in response to the measured strength ofvoltages induced by the MRI switched gradient fields such that acombination of the MRI switched gradient field induced currents producedby the number of operatively connected switches provide a combinedcurrent that is opposite to that which would be induced by the MRIswitched gradient fields in the medical device so as to reduce voltagesinduced by the MRI switched gradient fields to a safe level. The coilsmay be curved in three different spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to a voltagecontrol unit that reduces the effects of MRI induced signals upon amedical device to a safe level. The voltage control unit includes threeorthogonally planar coil, each coil generating a MRI switched gradientfield induced current; a sensor to measure a strength of voltagesinduced by the MRI switched gradient fields; and a switching device,operatively connected to the sensor and plurality of coils, tooperatively connect a number of the plurality of coils in response tothe measured strength of voltages induced by the MRI switched gradientfields such that a combination of the MRI switched gradient fieldinduced currents produced by the number of operatively connectedswitches provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical device soas to reduce voltages induced by the MRI switched gradient fields to asafe level.

In a further embodiment, the present invention is directed to a voltagecontrol unit that reduces the effects of MRI induced signals upon amedical device to a safe level. The voltage control unit includes aplurality of coils, each coil generating a MRI switched gradient fieldinduced current; a transceiver to receive a signal indicating a numberof coils to be connected; and a switching device, operatively connectedto the transceiver and the coils, to operatively connect a number of thecoils in response to the received signal indicating the number of coilsto be connected such that a combination of the MRI switched gradientfield induced currents produced by the number of operatively connectedswitches provide a combined current that is opposite to that which wouldbe induced by the MRI switched gradient fields in the medical deviceelectrical lead so as to reduce voltages induced by the MRI switchedgradient fields to a safe level. The coils may be curved in threedifferent spatial directions.

The embodiment may further include an orientation subsystem for changinga spatial orientation of the coil to modify the strength of the MRIswitched gradient field induced current.

In a further embodiment, the present invention is directed to a voltagecontrol unit that reduces the effects of MRI induced signals upon amedical device to a safe level. The voltage control unit includes threeorthogonally planar coil, each coil generating a MRI switched gradientfield induced current; a transceiver to receive a signal indicating anumber of coils to be connected; and a switching device, operativelyconnected to the transceiver and the coils, to operatively connect anumber of the coils in response to the received signal indicating thenumber of coils to be connected such that a combination of the MRIswitched gradient field induced currents produced by the number ofoperatively connected switches provide a combined current that isopposite to that which would be induced by the MRI switched gradientfields in the medical device electrical lead so as to reduce voltagesinduced by the MRI switched gradient fields to a safe level.

In a further embodiment, the present invention is directed to a lead formedical applications that reduces the effects of MRI induced signals toa safe level. The lead includes two coiled conductive strands forming aspring-like configuration such that current flows over a surfacethereof, through contact points between adjacent loops of the coiledconductive strands, and an insulating coating formed over a portion ofthe two coiled conductive strands such that an inline inductive elementis formed, the current flowing along a curvature of the two coiledconductive strands in the insulating coated portion of two coiledconductive strands.

This embodiment may further include a ferrite material positioned in theportion of the two-coiled conductive strands having the insulatingcoating formed thereon.

In a further embodiment, the present invention is directed to a lead formedical applications that reduces the effects of MRI induced signals toa safe level. The lead includes two coiled conductive strands forming aspring-like configuration such that current flows over a surfacethereof, through contact points between adjacent loops of the coiledconductive strands, and an adjustable resistive material formed over aportion of the two coiled conductive strands such that an inlineinductive element is formed, the current flowing along a curvature ofthe two coiled conductive strands in the adjustable resistive materialportion of two coiled conductive strands, an inductance of the inlineinductive element being adjusted by adjusting the resistive propertiesof the adjustable resistive material.

This embodiment may further include a ferrite material positioned in theportion of the two-coiled conductive strands having the insulatingcoating formed thereon.

In a further embodiment, the present invention is directed to a voltagecompensation unit for reducing the effects of induced voltages upon amedical device to a safe level. The voltage compensation unit includes aconnection device to provide an electrical connection to the medicaldevice; a sensing circuit to voltages of conductive components in themedical device; and a compensation circuit, operatively connected to thesensing circuit and responsive thereto, to provide opposing voltages tothe medical device to reduce the effects of induced voltages caused bychanging magnetic fields.

This embodiment may also include a power supply, such as a battery or aconnection to an external power source. The embodiment may include asecond sensing circuit to detect the changing magnetic fields and acompensation circuit, connected to the second sensing circuit andresponsive thereto, to synchronize application of the opposing voltagesto the medical device with the sensed changing magnetic fields.

In a further embodiment, the present invention is directed to a voltagecompensation unit for reducing the effects of induced voltages upon amedical device to a safe level. The voltage compensation unit includes aconnection device to provide an electrical connection to the medicaldevice; a sensing circuit to detect changing magnetic fields; and acompensation circuit, operatively connected to the sensing circuit andresponsive thereto, to synchronize application of opposing voltages tothe medical device with the sensed changing magnetic fields, theopposing voltages reducing the effects of induced voltages caused by thechanging magnetic fields.

This embodiment may also include a power supply, such as a battery or aconnection to an external power source.

In a further embodiment, the present invention is directed to a voltagecompensation unit for reducing the effects of induced voltages upon amedical device to a safe level. The voltage compensation unit includes aconnection device to provide an electrical connection to the medicaldevice; a communication circuit, communicatively linked to a MRI system,to receive information associated with a start and end of an applicationof changing magnetic fields produced by the MRI system; and acompensation circuit, operatively connected to the communication circuitand responsive thereto, to synchronize application of opposing voltagesto the medical device with the sensed changing magnetic fields, theopposing voltages reducing the effects of induced voltages caused by thechanging magnetic fields.

This embodiment may also include a power supply, such as a battery or aconnection to an external power source. The communication circuit mayreceive information associated with field strengths to be applied by theMRI system, and the compensation circuit would apply opposing voltagesin accordance with communicated applied field strengths. Thecommunication circuit may receive the information through electricalwires, coaxial wires, shielded wires, optical fibers, and/or aradio-frequency transmitter/receiver.

In a further embodiment, the present invention is directed to a voltagecompensation unit for reducing the effects of induced voltages upon amedical device to a safe level. The voltage compensation unit includes aconnection device to provide an electrical connection to the medicaldevice; a communication circuit, communicatively linked to a MRI system,to receive information associated with a start and end of an applicationof changing magnetic fields produced by the MRI system; and acompensation circuit, operatively connected to the communication circuitand responsive thereto, to apply opposing voltages to the medicaldevice, the opposing voltages reducing the effects of induced voltagescaused by the changing magnetic fields.

This embodiment may also include a power supply, such as a battery or aconnection to an external power source. The communication circuit mayreceive information associated with field strengths to be applied by theMRI system, and the compensation circuit would apply opposing voltagesin accordance with communicated applied field strengths. Thecommunication circuit may receive the information through electricalwires, coaxial wires, shielded wires, optical fibers, and/or aradio-frequency transmitter/receiver.

While various examples and embodiments of the present invention havebeen shown and described, it will be appreciated by those skilled in theart that the spirit and scope of the present invention are not limitedto the specific description and drawings herein, but extend to variousmodifications and changes.

1. A voltage compensation unit for reducing the effects of inducedvoltages upon a device to a safe level, comprising: a sensing circuit tosense voltages induced in conductive components of the device, thevoltages being induced by changing magnetic fields; and a compensationcircuit, operatively connected to said sensing circuit and responsivethereto, to provide opposing voltages to the device to reduce theeffects of induced voltages caused by changing magnetic fields.
 2. Thevoltage compensation unit as claimed in claim 1, further comprising apower supply.
 3. The voltage compensation unit as claimed in claim 2,wherein said power supply is a battery.
 4. The voltage compensation unitas claimed in claim 1, further comprising: a second sensing circuit todetect the changing magnetic fields; said compensation circuit,operatively connected to said second sensing circuit and responsivethereto, to synchronize application of the opposing voltages to thedevice with the sensed changing magnetic fields.
 5. The voltagecompensation unit as claimed in claim 1, wherein said compensationcircuit is shielded from the changing magnetic fields.
 6. The voltagecompensation unit as claimed in claim 1, further comprising a connectiondevice to provide an electrical connection between said sensing circuitand said compensation circuit and the device.
 7. The voltagecompensation unit as claimed in claim 6, wherein said connection deviceprovides multiple electrical connections between said sensing circuitand said compensation circuit and the device.
 8. The voltagecompensation unit as claimed in claim 7, wherein said connection deviceis electrically connected to the device at unequally spaced intervals.9. The voltage compensation unit as claimed in claim 7, wherein aportion of said multiple electrical connections of said connectiondevice is electrically connected to non-resonance node points of thedevice.
 10. A voltage compensation unit for reducing the effects ofinduced voltages upon a tissue invasive medical tool to a safe level,comprising: a sensing circuit to sense voltages induced in conductivecomponents of the medical tool, the voltages being induced by changingmagnetic fields; a compensation circuit, operatively connected to saidsensing circuit and responsive thereto, to provide opposing voltages tothe medical tool to reduce the effects of induced voltages caused bychanging magnetic fields; and a connection device to provide anelectrical connection between said sensing circuit and said compensationcircuit and the medical tool.
 11. The voltage compensation unit asclaimed in claim 10, wherein said connection device provides multipleelectrical connections along the medical tool.
 12. The voltagecompensation unit as claimed in claim 11, wherein said connection deviceis electrically connected to the medical tool at unequally spacedintervals.
 13. The voltage compensation unit as claimed in claim 11,wherein a portion of said multiple electrical connections of saidconnection device is electrically connected to non-resonance node pointsof the medical tool.
 14. The voltage compensation unit as claimed inclaim 10, further comprising: a second sensing circuit to detectchanging magnetic fields; said compensation circuit, operativelyconnected to said second sensing circuit and responsive thereto, tosynchronize application of opposing voltages to the medical tool withthe sensed changing magnetic fields, said opposing voltages reducing theeffects of induced voltages caused by the changing magnetic fields. 15.The voltage compensation unit as claimed in claim 10, wherein saidcompensation circuit is shielded from the changing magnetic fields. 16.A voltage compensation unit for reducing the effects of induced voltagesupon a device to a safe level, comprising: a communication circuit,communicatively linked to a MRI system, to receive, from the MRI system,information associated with a start and end of an application ofchanging magnetic fields produced by the MRI system; and a compensationcircuit, operatively connected to said communication circuit andresponsive thereto, to synchronize application of opposing voltages tothe device with the start and end of the application of the changingmagnetic fields produced by the MRI system, said opposing voltagesreducing the effects of induced voltages caused by the changing magneticfields.
 17. The voltage compensation unit as claimed in claim 16,further comprising a connection device to provide an electricalconnection between said compensation circuit and the device.
 18. Thevoltage compensation unit as claimed in claim 17, wherein saidconnection device provides multiple electrical connections between saidcompensation circuit and the device.
 19. The voltage compensation unitas claimed in claim 18, wherein said connection device is electricallyconnected to the device at unequally spaced intervals.
 20. The voltagecompensation unit as claimed in claim 18, wherein a portion of saidmultiple electrical connections of said connection device iselectrically connected to non-resonance node points of the device. 21.The voltage compensation unit as claimed in claim 16, wherein saidcommunication circuit receives information associated with MRI scanpulse sequences to be applied by the MRI system; said compensationcircuit applies opposing voltages in accordance with communicatedapplied MRI scan pulse sequences.
 22. The voltage compensation unit asclaimed in claim 16, wherein said communication circuit receivesinformation associated with timing of application of fields and pulseshapes thereof to be applied by the MRI system; said compensationcircuit applies opposing voltages in accordance with communicated timingof applied fields and pulse shapes thereof.
 23. The voltage compensationunit as claimed in claim 16, wherein said communication circuit receivesinformation associated with pulse shapes of a field to be applied by theMRI system; said compensation circuit applies opposing voltages inaccordance with communicated applied pulse shapes.
 24. The voltagecompensation unit as claimed in claim 16, wherein said communicationcircuit receives the information through electrical wires.
 25. Thevoltage compensation unit as claimed in claim 16, wherein saidcommunication circuit receives the information through coaxial wires.26. The voltage compensation unit as claimed in claim 16, wherein saidcommunication circuit receives the information through shielded wires.27. The voltage compensation unit as claimed in claim 16, wherein saidcommunication circuit receives the information through optical fibers.28. The voltage compensation unit as claimed in claim 16, wherein saidcommunication circuit receives the information through a radio-frequencytransmitter/receiver.
 29. The voltage compensation unit as claimed inclaim 16, wherein said communication circuit receives the informationthrough an acoustic transmitter/receiver.
 30. A voltage compensationunit for reducing the effects of induced voltages upon a device to asafe level, comprising: a communication circuit, communicatively linkedto a MRI system, to receive, from the MRI system, information associatedwith a start and end of an application of changing magnetic fieldsproduced by the MRI system; and a compensation circuit, operativelyconnected to said communication circuit and responsive thereto, to applyopposing voltages to the device, said opposing voltages reducing theeffects of induced voltages caused by the changing magnetic fields. 31.The voltage compensation unit as claimed in claim 30, further comprisinga connection device to provide an electrical connection between saidcompensation circuit and the device.
 32. The voltage compensation unitas claimed in claim 31, wherein said connection device provides multipleelectrical connections between said compensation circuit and the device.33. The voltage compensation unit as claimed in claim 32, wherein saidconnection device is electrically connected to the device at unequallyspaced intervals.
 34. The voltage compensation unit as claimed in claim32, wherein a portion of said multiple electrical connections of saidconnection device is electrically connected to non-resonance node pointsof the device.
 35. The voltage compensation unit as claimed in claim 30,wherein said communication circuit receives information associated withMRI scan pulse sequences to be applied by the MRI system; saidcompensation circuit applies opposing voltages in accordance withcommunicated applied MRI scan pulse sequences.
 36. The voltagecompensation unit as claimed in claim 30, wherein said communicationcircuit receives information associated with timing of application offields and pulse shapes thereof to be applied by the MRI system; saidcompensation circuit applies opposing voltages in accordance withcommunicated timing of applied fields and pulse shapes thereof.
 37. Thevoltage compensation unit as claimed in claim 30, wherein saidcommunication circuit receives information associated with pulse shapesof a field to be applied by the MRI system; said compensation circuitapplies opposing voltages in accordance with communicated applied pulseshapes.
 38. The voltage compensation unit as claimed in claim 30,wherein said communication circuit receives the information throughelectrical wires.
 39. The voltage compensation unit as claimed in claim30, wherein said communication circuit receives the information throughcoaxial wires.
 40. The voltage compensation unit as claimed in claim 30,wherein said communication circuit receives the information throughshielded wires.
 41. The voltage compensation unit as claimed in claim30, wherein said communication circuit receives the information throughoptical fibers.
 42. The voltage compensation unit as claimed in claim30, wherein said communication circuit receives the information througha radio-frequency transmitter/receiver.
 43. The voltage compensationunit as claimed in claim 30, wherein said communication circuit receivesthe information through an acoustic transmitter/receiver.
 44. A voltagecompensation unit for reducing the effects of induced voltages upon adevice having a single wire line, the single wire line having a balancedcharacteristic impedance, comprising: a tunable compensation circuit,operatively connected to the wire line, to apply variable supplementalimpedance to the wire line, said variable supplemental impedance causingthe characteristic impedance of the wire line to become unbalanced,thereby reducing the effects of induced voltages caused by changingmagnetic fields.
 45. The voltage compensation unit as claimed in claim44, wherein said tunable compensation circuit is a plurality of variablecapacitors.
 46. The voltage compensation unit as claimed in claim 44,wherein said tunable compensation circuit is a balun.
 47. The voltagecompensation unit as claimed in claim 44, wherein said tunablecompensation circuit is an IF amplifier, said IF amplifier automaticallyapplying supplemental impedance to the wire line to cause thecharacteristic impedance of the wire line to become unbalanced.
 48. Thevoltage compensation unit as claimed in claim 44, wherein said tunablecompensation circuit is manually tunable to change an amount of saidsupplemental impedance being applied to the wire line.
 49. The voltagecompensation unit as claimed in claim 2, wherein said power supply is aconnection to an external power source.